1. Field of the Invention
The present invention relates to a control circuit for a light-emitting device, in particular, the invention relates to a control circuit to control a temperature of the light-emitting device such as semiconductor laser diode, by monitoring the temperature thereof, to an optimum condition.
2. Related Prior Art
Various techniques to optimize the temperature of the laser diode by monitoring the temperature thereof have been disclosed in the U.S. Pat. No. 4,631,728 and Japanese patent published as S59-224931.
The control circuit recited in the former material is that the temperature of the laser diode is kept constant by a thermoelectric device, i.e., a signal corresponding to the temperature of the laser diode sensed by a temperature sensor is amplified and thus amplified signal is compared with a reference voltage by an integration circuit. An output from the integration circuit controls a pulse width modulation circuit (hereinafter referred as PWM), and the PWM circuit drives a switching transistor that varies a pulse width of a supply voltage via a filtering circuit. Thus filtered supply voltage is provided to a mode controller. The mode controller, under the control of the integration circuit, switches a direction of current flowing. The supply voltage is applied to the thermoelectric device to keep the temperature of the laser diode constant.
The control circuit according to the latter reference includes (1) a thermoelectric conversion block where a temperature of a thermoelectric device, capable of both heating and cooling by the direction of current flowing, is detected and converted to an electric signal, (2) a comparing block for comparing thus converted electric signal with a reference signal, and (3) a voltage-to-current conversion block for driving the thermoelectric device in a current mode. The voltage-to-current conversion block includes an npn-transistor and a pnp-transistor, and is connected in its input terminal to an output terminal of the comparing block. Further, between the input terminal of the voltage-to-current conversion block and bases of both the npn-transistor and the pnp-transistor are provided level shift circuits, respectively.
Since the supply voltage is controlled by the PWM mode, the former control circuit has an advantage in power efficiency. Operating under the ideal condition with no loss in devices, the power efficiency of 100% would be achieved. Practically, the power efficiency from 85% to 86%, as denoted by a symbol B in FIG. 3 later shown, would be achievable due to impedance of the switching transistor and inductors. Further, the integration circuit is necessary to reduce a switching noise as shown in FIG. 1. When the driving current is flew in bi-directional for heating and cooling, two integration circuits must be installed. In particular, when a large driving current is necessary, integration circuits that include a large inductor and a large capacitor would be necessary, thereby causing an increment of the size of the circuit assembly.
On the other hand, in the control circuit of the latter reference, since the current flowing in the thermoelectric controller is controlled in linear, as shown in FIG. 4, the voltage between 0 to the power supply voltage VCC is generated between the collector and the emitter of the bipolar transistor or between the drain and the source of the FET. This voltage drop causes the heat generation of the transistor or the FET. Therefore, the transistor or the FET dissipates heat greater than what is consumed by the thermoelectric controller, thereby reducing the power efficiency. The thermoelectric controller in this case, the switching noise does not occur in principle because the controller is not operated in switching mode.
Therefore, one object of the present invention is, by solving both problems appeared in the former reference and also in the latter reference, to provide a temperature control circuit for a light-emitting device, in which the circuit has the improved power efficiency and does not show switching noise without enlarging the size of the circuit.